Land application equipment used on animal production farms must be field calibrated or evaluated in accordance with existing design charts and tables according to state rules that went into effect September 1, 1996. Technical Specialist certifying waste management plans after September 1, 1996, must also certify that operators have been provided calibration and adjustment guidance for all land application equipment. The rules apply to irrigation systems as well as all other types of liquid, slurry, or solid application equipment.

Information presented in manufacturers’ charts are based on average operating conditions for relatively new equipment. Discharge rates and application rates change over time as equipment ages and components wear. As a result, equipment should be field calibrated regularly to ensure that application rates and uniformity are consistent with values used during the system design and given in manufacturers’ specifications. Field calibration involves collection and measurement of the material being applied at several locations in the application area. This publication contains step-by-step guidelines for field calibration of center pivot and linear move irrigation systems.

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Operating an irrigation system differently than assumed in the design will alter the application rate, uniformity of coverage, and subsequently the application uniformity. Operating with excessive pressure results in smaller droplets, greater potential for drift, and accelerates wear of the sprinkler nozzle. Pump wear tends to reduce operating pressure and flow.

With continued use, nozzle wear results in an increase in the nozzle opening, which will increase the discharge rate while decreasing the wetted diameter. Clogging of nozzles or crystallization of main lines can result in increased pump pressure but reduced flow at the sprinkler. Plugged intakes will reduce operating pressure. An operating pressure below design pressure greatly reduces the coverage diameter and application uniformity. Field calibration helps ensure that nutrients from animal waste are applied uniformly and at proper rates.

The calibration of a center pivot and linear move irrigation systems involves setting out collection containers, operating the system, measuring the amount of wastewater collected in each container, then computing the average application volume and application uniformity.

An in-line flow meter installed in the main irrigation line provides a good estimate of the total volume pumped from the lagoon during each irrigation cycle. The average application depth can be determined by dividing the pumped volume by the application area. The average application depth is computed from the formula:

\(\mathrm{Average\ application\ depth\ (inches)\ =\ }\frac{Volume\ pumped\ (gallons)}{27,154\ (gal/ac-in)\ \times\ Application\ area\ (acres)}\)

The average application depth is the average amount applied throughout the field. Unfortunately, sprinklers do not apply the same depth of water throughout their wetted area. Under normal operating conditions, application depth decreases towards the outer perimeter of the wetted diameter. Sprinkler systems are designed to have overlap to compensate for the declining application along the outer perimeter. When operated at the design pressure, this overlap results in acceptable application uniformity.

When operated improperly, well-designed systems will not provide acceptable application uniformity. For example, if the pressure is too low, the application depth will be several times higher near the center of sprinkler and water will not be thrown as far from the sprinkler as indicated in manufacturers’ charts. Even through the average application depth may be acceptable, some areas receive excessively high application while others receive no application at all.

When applying wastewater high in nutrients, it is important to determine the application uniformity. Collection containers distributed throughout the application area must be used to evaluate application uniformity.

Many types of containers can be used to collect flow and determine the application uniformity. Standard rain gauges work best and are recommended because they already have a graduated scale from which to read the application depth.

Pans, plastic buckets, jars, or anything with a uniform opening and cross section can be used provided the container is deep enough (at least 4 inches) to prevent splash and excessive evaporation, and the liquid collected can be easily transferred to a scaled container for measuring. All containers should be the same size and shape.

All collection containers should be set up at the same height relative to the height of sprinkler nozzle (discharge elevation). Normally, the top of each container should be no more than 36 inches above the ground and no more than 36 inches below the sprikler or nozzler discharge elevation. Collectors should be located so that there is no interference from the crop. The crop canopy should be trimmed to preclude interference or splash into the collection container.

Calibration should be performed during periods of low evaporation. Best times are before 10 a.m. or after 4 p.m. on days with light wind (less than 5 miles). On cool, cloudy days the calibration can be performed anytime during the day when wind velocity is less than 5 mph.

The volume (depth) collected during calibration should be read as soon as the system completely passes over the row of collection containers to minimize evaporation from gauges. Where a procedure must be performed more than once, containers should be read and values recorded immediately after each different setup.

Center pivot and linear move irrigation systems are calibrated by placing a row (transect) of collection containers parallel to the system as shown in Figure 1 and Figure 2. Two or more rows increase the accuracy of the calibration.

For center pivot systems with multiple towers, place the first collection container beside the first moving tower (140 to 180 feet from the boss tower (pivot point)). This will miss the area between the boss and first tower, but it is necessary to omit this area because too much time is required to operate the system through this zone. The area missed will be less than 3 acres and will usually represent less than 10 percent of a typical sized system. If the system has only one moving tower, place the first container 100 feet from the boss tower. Place containers equally spaced to the end of the system.

For lateral move systems, place containers throughout the entire length of the system.

Containers should be spaced no further apart than one-half the wetted diameter of rotary impact sprinklers, 1/4 the diameter of gun sprinklers, or 50 feet, whichever is less. On systems with spray nozzles, collection containers should be spaced no further than 30 feet. A 20 to 25 foot spacing is generally recommended for all types of sprinklers which will result in 6 to 8 collection containers between each tower. Collection containers should be placed such that they intercept discharge from a range of lateral distances from the sprinkler (mid-point, quarter point, directly under sprinkler, etc.). This can be accomplished by selecting a catch can spacing different from a multiple of the sprinkler spacing along the lateral.

Where end guns are used, the transect of collection containers should extend beyond the throw of the gun.

The system should be operated so that the minimum travel distance exceeds the sprinkler wetted diameter for the container closest to the boss tower. Application volumes should be read as soon as all gauges stop being wetted.

Figure 1. General layout and orientation of collection gauges for calibration of a center pivot or linear move irrigation system.

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Figure 1. General layout and orientation of collection gauges for calibration of a center pivot or linear move irrigation system.

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Figure 2. Accounting for overlap when calibrating a hard hose traveler system.

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Figure 2. Accounting for overlap when calibrating a hard hose traveler system.

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Date:________

Landowner:________

Farm No. :________

*Absolute value; treat all values as positive. ↲

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